We propose to study cortical organization and reorganization at the level of neuronal networks and assemblies, using both experimental and theoretical methods. The experimental vehicle will be rat somatosensory cortex subjected to local electrical stimulation; this forces rapid changes in somatic map boundaries. During such changes we propose to make extracellular region. These measurements and their subsequent interpretation in terms of neuronal assembly processes will rest on recording technologies and analytic mathematics that have largely been developed by our laboratory over the last decade. The proposed theoretical work will examine computer simulations of neuronal networks arranged to reproduce the changes in map magnification and boundaries which underlie the experimental work. As information about "effective connectivity" and neuronal assembly properties emerges from the experiments, these new constraints will be incorporated into the ongoing modeling. The models will analysis techniques, and are likely to suggest additional experimental measurements. The overall project should give considerable insight into actual and possible mechanisms for cortical modularity and reorganization; we will here have, for the first time, direct experimental access to the details of the process by which the brain reallocates its computational resources.